1. Field of the Invention
This invention relates generally to wind shear detecting and warning systems of the type that provide an alert or a warning to the pilot of an aircraft in the event of a wind shear condition. More particularly, the present invention relates to a wind shear alerting and warning system that has variable warning parameters that vary as a function of the flight path of the aircraft to provide an alert or a warning that varies as a function of flight path. An alert is provided prior to the warning if the flight path is such that the aircraft is particularly vulnerable to wind shears.
2. Prior Art
Many airborne wind shear warning systems are known. Many of such systems compare inertially derived accelerations or velocities with airmass derived accelerations or velocities to provide a wind shear warning to the pilot of the aircraft when the inertially derived and airmass derived accelerations or velocities differ by an amount sufficient to indicate a wind shear condition. Examples of such systems are disclosed in U.S. Pat. No. 4,725,811, entitled "WIND SHEAR DETECTING AND ALERTING SYSTEM", filed on Feb. 13, 1986 by Hans R. Muller, and assigned to the same assignee as the assignee of the present invention. Other wind shear detecting systems are disclosed in U.S. Pat. Nos. 4,012,713 and 4,079,905.
In the aforesaid systems, the level of wind shear at which the warning is given to the pilot must be carefully selected. In normal aircraft operation, there are many low level wind shears that are routinely compensated for by the pilot. In many instances, the pilot automatically compensates for such wind shears while maintaining his flight path, for example, staying on the glide slope, and may not even be aware that a wind shear condition was encountered. Consequently, present day wind shear warning systems are designed such that they do not provide a warning in response to low level wind shears because such warnings would be irritating and distracting to the pilot. Also, such warnings would be considered to be nuisance warnings, and consequently, many pilots would simply ignore such warnings or even turn off the warning system.
To avoid such nuisance warnings, the warning thresholds of present-day systems are set at a relatively high level. This level is sufficiently high that a substantial proportion of the performance capability and energy of the aircraft is required to escape the shear. Typically, the warning threshold is set for approximately 120 to 150 milli-G's or 2 to 3 knots per second of tail shear or vertical shear. Tail shear is defined as a decreasing head wind or an increasing tail wind or a change from a head wind to a tail wind in magnitude or direction. Vertical shear is defined as an increase in vertical downdraft or decrease in updraft. Also, the magnitude of tail shear must exceed the threshold level for a predetermined time, typically on the order of 3 to 5 seconds, before the warning is given. While a system having a threshold of negative 2 to 3 knots per second (i.e., 2 to 3 knots per second of tail shear) would give a warning in time to permit the pilot to execute a successful escape maneuver in most instances, a study of previous wind shear accidents has indicated that wind shears having magnitudes of below the threshold value have also contributed to accidents, particularly if the aircraft was flying an abnormal flight path. Such low level wind shears would not trigger a wind shear warning in systems having a threshold set at 2 to 3 knots per second of negative wind shear.
A system that utilizes a combination of flight parameters to generate a warning is disclosed in U.S. Pat. No. 4,189,777. In the system disclosed in the '777 patent there is provided a Mode 1 warning system that generates a warning if the barometric altitude sink rate is excessive for the radio altitude at which the aircraft is flying. The sink rate required to generate a warning is varied as a function of the rate of change of airspeed. In alternative embodiments, the Mode 1 warning criteria are varied as a function of the difference between the rate of change of airspeed and the rate of change of ground speed, or as a function of the difference between airspeed rate and a longitudinal accelerometer signal. However, no warning was given to the pilot that wind shear was present.